1. Field of the Invention
The present invention relates to an electronic distance meter having a sighting telescope and a focus detection device for detecting a focus state of the sighting telescope.
2. Description of the Related Art
When a surveyor measures the distance between two points, an electronic distance meter (EDM) is generally used. An electronic distance meter calculates the distance via the phase difference between a projecting light and a reflected light and via the initial phase of an internal reference light, or via the time difference between the projecting light and the reflected light.
A typical electronic distance meter is provided, behind the objective lens of a sighting telescope thereof, with a light transmitting mirror positioned on the optical axis of the sighting telescope to project the measuring light toward a target through the center of the entrance pupil of the objective lens of the sighting telescope. The light which is reflected by the target to be passed through the objective lens of the sighting telescope passes the peripheral space of the light transmitting mirror to be captured via a wavelength selection filter and a light receiving mirror.
In such an electronic distance meter, the light which is reflected by the target and passed through the objective lens of the sighting telescope is interrupted by the aforementioned light transmitting mirror by a greater amount as the target is closer to the electronic distance meter. If the light which is reflected by the target and passed through the objective lens of the sighting telescope is interrupted by the light transmitting mirror by a great amount, the light amount of the incident light upon the aforementioned light receiving mirror decreases, which deteriorates the precision in measuring the object distance. If the target is very close to the electronic distance meter, the light which is reflected by the target and passed through the objective lens of the sighting telescope may not be incident on a light-receiving element (photo-receiver) at all, which makes it impossible to perform a distance measuring operation. To prevent these problems from occurring, various methods have been proposed.
Advancements have been made in the development of surveying instruments provided with a sighting telescope having an autofocus system, wherein phase-difference detection type autofocus system is widely used in the autofocus therefor. With this system, an in-focus state is detected based on the correlation between two images formed by two light bundles which are respectively passed through two different pupil areas upon passing through different portions of an objective lens of the sighting telescope to bring the sighting telescope into focus in accordance with the detected in-focus state.
However, in the case where a phase-difference detection type autofocus system is incorporated into an electronic distance meter, it is difficult to overcome the problem of a decrease in the amount of light incident upon the aforementioned light receiving element (photo-receiver) when the target is close to the electronic distance meter and also the problem of deterioration of the precision in measuring the object distance that is caused by a decrease in the amount of light incident upon the light receiving element.
The present invention has been devised in view of the problems noted above, and accordingly, an object of the present invention is to provide an electronic distance meter having a sighting telescope and a focus detection device for detecting a focus state of the sighting telescope, wherein the electronic distance meter is free from the problem of the amount of light incident upon a light-receiving element decreasing when the target is close to the electronic distance meter, and further free from deterioration of precision in measuring the object distance.
To achieve the objects mentioned above, according to an aspect of the present invention, an electronic distance meter is provided, including a sighting telescope having an objective lens for sighting an object; a reflection member positioned behind the objective lens on an optical axis of the objective lens; an optical distance meter which includes a light-transmitting optical system for transmitting a measuring light via the reflection member and the objective lens, and a light-receiving optical system for receiving a portion of the measuring light which is reflected by the object, subsequently passed through the objective lens and not obstructed by the reflection member; a phase-difference detection focus detecting device which detects a focus state from a correlation between a pair of images respectively formed by two light bundles which are respectively passed through two different pupil areas on the objective lens; and at least one optical element, positioned outside the two different pupil areas, to deflect the portion of the measuring light which is reflected by the object, subsequently passed through the objective lens and not obstructed by the reflection member, toward an optical axis of the light-receiving optical system.
Preferably, the at least one optical element has at least one function of reflecting, refracting, and diffracting light which is incident thereon.
Preferably, the at least one optical element is positioned between the two different pupil areas.
In an embodiment, the light-receiving optical system includes a wavelength selection mirror positioned behind the reflection member, the at least one optical element being positioned between two different pupil areas on the wavelength selection mirror.
In an embodiment, two of the optical elements are positioned on opposite sides of the optical axis of the light-receiving optical system.
In an embodiment, the light-receiving optical system includes a light-receiving element, the at least one optical element being designed so that a sufficient amount of the portion of measuring light which is reflected by the object is received by the light-receiving element when the object is positioned at a short distance.
In an embodiment, the at least one optical element is a Fresnel mirror which includes a series of mirror sections.
Preferably, the mirror sections of the Fresnel mirror function as either an inclined plane mirror or a concave mirror.
In an embodiment, the reflection member is made of a parallel-plate mirror having front and rear surfaces parallel to each other, the reflection member being inclined to the optical axis.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-154256 (filed on May 25, 2000) which is expressly incorporated herein by reference in its entirety.